Transcript Linking Asteroids and Meteorites through Reflectance

Conversations with the Earth
Tom Burbine
[email protected]
Quiz on Thursday
• Sun
• Hertzsprung-Russell Diagram
• Death of stars
Main Sequence
• Is not an evolutionary track
– Stars do not evolve on it
• Stars stop on the main sequence and spend most
of their lives on it
Sun ends it time on the main sequence
• When the core hydrogen is depleted, nuclear
fusion stops
• The core pressure can no longer resist the crush of
gravity
• Core shrinks
Why does the star expand?
• The core is made of helium
• The surrounding layers are made of hydrogen
And ..
• Gravity shrinks the inert helium core and
surrounding shell of hydrogen
• The shell of hydrogen becomes hot for fusion
• This is called hydrogen-shell burning
And …
• The shell becomes so hot that its fusion rate is
higher than the original core
• This energy can not be transported fast enough to
surface
• Thermal pressure builds up and the star expands
And ..
•
•
•
•
More helium is being created
Mass of core increases
Increases its gravitational pull
Increasing the density and pressure of this region
When
• When helium core reaches 100 million Kelvin,
• Helium can fuse into a Carbon nucleus
Helium Flash
• The rising temperature in the core causes the
helium fusion rate to rocket upward
• Creates a lot of new energy
However
• The core expands
• Which pushes the hydrogen-burning shell
outwards
• Lowering the hydrogen-burning shell’s
temperature
And
• Less energy is produced
• Star starts to contract
Now
• In the core, Helium can fuse to become Carbon
(and some Oxygen)
• Star contracts
• Helium fusion occurs in a shell surrounding the
carbon core
• Hydrogen shell can fuse above the Helium shell
• Inner regions become hotter
• Star expands
http://upload.wikimedia.org/wikipedia/commons/8/8d/Triple-Alpha_Process.png
• Some carbon fuses with He to form Oxygen
• 12C + 4He → 16O + gamma ray
• Harder to fuse Oxygen with Helium to produce
Neon
Planetary Nebulae
• There is a carbon core and outer layers are ejected
into space
• The core is still hot and that ionizes the expanding
gas
Planetary Nebulae
White Dwarf
• The remaining core becomes a white dwarf
• White dwarfs are usually composed of carbon and
oxygen
• Oxygen-neon-magnesium white dwarfs can also
form
• Helium white dwarfs can also form
High-Mass Stars
• The importance of high-mass stars is that they
make elements heavier than carbon
• You need really hot temperatures which only
occur with the weight of a very high-mass star
Stages of High-Mass Star’s Life
• Similar to low-mass star’s
• Except a high-mass star can continue to fuse
elements
• When the fusion ceases, the star becomes a
supernova
• Supernova is a huge explosion
Fusion
• The temperatures of high-mass stars in its latestage of life can reach temperatures above 600
million Kelvin
• Can fuse Carbon and heavier elements
• Helium Capture can also occur where Helium can
be fused into heavy elements
“Deaths” of Stars
• White Dwarfs
• Neutron Stars
• Black Holes
White Dwarfs
• White Dwarfs is the core left over when a star can
no longer undergo fusion
• Most white dwarfs are composed of carbon and
oxygen
• Very dense
– Some have densities of 3 million grams per cubic
centimeter
– A teaspoon of a white dwarf would weigh as much as
an elephant
White Dwarfs
• Some white dwarfs have the same mass as the
Sun but slightly bigger than the Earth
• 200,000 times as dense as the earth
White Dwarfs
• Collapsing due to gravity
• The collapse is stopped by electron degeneracy
pressure
Electron Degeneracy Pressure
• No two electrons can occupy the same quantum
state
The Sun
• Will end up as a White Dwarf
Neutron Star
•
•
•
•
Neutron stars are usually 10 kilometers acroos
But more massive than the Sun
Made almost entirely of neutrons
Electrons and protons have fused together
How do you make a neutron star?
• Remnant of a Supernova
Supernova
• A supernova is a stellar explosion.
• Supernovae are extremely luminous and cause a
burst of radiation that often briefly outshines an
entire galaxy, before fading from view over
several weeks or months.
Type Ia Supernova
Type II Supernova
This stops with Iron
• Fusion of Iron with another element does not
release energy
• Fission of Iron does not release energy
• So you keep on making Iron
Initially
• Gravity keeps on pulling the core together
• The core keeps on shrinking
• Electron degeneracy keeps the core together for
awhile
Then
• The iron core becomes too massive and collapses
• The iron core becomes neutrons when protons and
electrons fuse together
Density of neutron star
• You could take everybody on Earth and cram
them into a volume the size of sugar cube
Explosion
• The collapse of the core releases a huge amount
of energy since the rest of the star collapses and
then bounces off the neutron core
• 1044-46 Joules
• Annual energy generation of Sun is 1034 Joules
How do we know there are neutron stars?
• The identification of Pulsars
• Pulsars give out pulses of radio waves at precise
intervals
Pulsars
• Pulsars were found at the center of supernovae
remnants
Pulsars
• Pulsars were interpreted as rotating neutron stars
• Only neutron stars could rotate that fast
• Strong magnetic fields can beam radiation out
Black Holes
• If a collapsing stellar core has a mass greater than
3 solar masses,
• It becomes a black hole
Black Hole
• After a supernova if all the outer mass of the star
is not blown off
• The mass falls back on the neutron star
• The gravity causes the neutron star to keep
contracting
Black Hole
• A black hole is a region where nothing can
escape, even light.
Event Horizon
• Event Horizon is the boundary between the inside
and outside of the Black Hole
• Within the Event Horizon, the escape velocity is
greater than the speed of light
• Nothing can escape once it enters the Event
Horizon
Black Hole Sizes
• A Black Hole with the mass of the Earth would
have a radius of 0.009 meters
• A Black Hole with the mass of the Sun would
have a radius of 3 kilometers
http://www.astronomynotes.com/evolutn/remnants.gif
Can you see a Black Hole?
No
• Black Holes do not emit any light
• So you must see them indirectly
• You need to see the effects of their gravity
Evidence
• The white area is
the core of a Galaxy
• Inside the core there
is a brown spiralshaped disk.
• It weighs a hundred
thousand times as
much as our Sun.
http://helios.augustana.edu/~dr/img/ngc4261.jpg
Evidence
• Because it is rotating we can measure its
radii and speed, and hence determine its
mass.
• This object is about as large as our solar
system, but weighs 1,200,000,000 times as
much as our sun.
• Gravity is about one million times as strong
as on the sun.
• Almost certainly this object is a black hole.
Any Questions?